The present invention is related to the field of integrated circuits that are influenced by temperature gradients and, more particularly, to integrated circuits that include temperature gradient canceling features.
Although the present invention applies to integrated circuits whose performance can be affected deleteriously by temperature gradients, the invention will be described in terms of a thermal accelerometer. Those of ordinary skill in the art can appreciate the adaptability of the thermal accelerometer application to other applications such as flow sensors, pressure sensors, opamps, voltage references, supply regulators, and the like.
Conventional thermal accelerometers, which include a heating element and thermopile pairs to determine acceleration by measuring changes in temperature of a fluid, are themselves highly sensitive to temperature gradients across the integrated circuit (“chip”). Indeed, an acceleration sensing device often cannot differentiate between an acceleration signal and a temperature gradient, which the sensor may interpret as an acceleration signal. As a result, temperature gradients across the chip can produce an offset shift.
Internal or systemic temperature gradient conditions that are caused by the internal workings and normal operation of the chip can be addressed in manufacture. However, when the chip is integrated into a system, e.g., on a printed circuit board (PCB), heat-generating components proximate the accelerometer can also cause problems. For example, during system start-up and before thermal equilibrium has been reached, significant thermal gradients can result due to the sequential timing of start-up and the varying warm-up rates of the individual components and devices making up the system. Temperature gradients can also remain after start-up due to the proximity of the chip to heat-generating devices, to cooling mechanisms that cool unevenly, and the like. Heat generation of different devices may also change with time, for example a circuit that is enabled then disabled or vice versa, cooling fans turning on and off, motors operating or not operating, and so forth. Those of ordinary skill in the art know that this problem is not unique to thermal accelerometers but exits for many sensing device or system, e.g., a flow sensor, a pressure sensor, and the like, that uses temperature and temperature differentials.
Unfortunately, temperature gradients resulting from external elements and stimuli, e.g., due to environmental conditions, having to do with the PCB, and the like, cannot be compensated for in manufacture. More particularly, a generic chip can be used in a multiplicity of applications, whose system designs are unknown to the chip designer, but which can create unique operating environments. As a result of unique temperature gradient conditions in these applications, the thermal accelerometer will exhibit offsets that differ from the value it was set for at the factory.
FIG. 1 depicts a thermal acceleration sensor and FIG. 2 depicts a thermal accelerometer integrating the thermal acceleration sensor in accordance with U.S. Pat. No. 7,305,881 commonly assigned to MEMSIC, Inc. of Andover, Mass., the assignee of the present invention.
Referring to FIG. 1, the thermal acceleration sensor 101 includes a substantially planar substrate 102, a cavity 103 formed in the substrate 102, a heater element 104 suspended over the cavity 103, a first pair of temperature sensing elements 106a-106b disposed along the x-axis, and a second pair of temperature sensing elements 107a-107b disposed along the y-axis. The thermal acceleration sensor 101 further includes a fluid disposed in the cavity 103 to allow convective heat transfer to occur in the vicinity of the cavity 103.
Each temperature sensing element of the temperature sensing element pairs 106a-106b and 107a-107b is disposed at substantially equal distances from the heater element 104. Furthermore, the heater element 104 is operative to produce a temperature gradient within the fluid that is symmetrical in both the x- and y-direction when the device is at rest. Accordingly, the symmetrical temperature gradients along the x- and y-axes cause the differential temperature between the temperature sensing element pairs 106a-106b and 107a-107b to be zero when the thermal acceleration sensor 101 is at rest.
In the event an accelerating force is applied to the sensor 101, for example, in the x-direction, the temperature distribution shifts, thereby allowing a non-zero differential temperature proportional to the magnitude of the applied acceleration to be detected by the temperature sensing elements 106a and 106b. Similarly, in the event an accelerating force is applied to the sensor 101 in the y-direction, the temperature distribution shifts to allow a non-zero differential temperature proportional to the magnitude of the applied acceleration to be detected by the temperature sensing elements 107a-107b. 
The thermal accelerometer 300 shown in FIG. 2 is structured and arrange to provide output voltages Vout,a and Vout,b representing magnitudes of acceleration in the directions of the x- and y-axes, respectively. The embodied thermal accelerometer 300 includes the thermal acceleration sensor 101 of FIG. 1, as well as heater control circuitry 318, amplification circuitry 314, and signal conditioning circuitry 360, which preferably are integrated on a single chip.
The foregoing design remains sensitive to thermal gradient along the sensitive (x- and y-) axes. More specifically, referring to FIG. 1, any temperature gradient along the North (N)-South (S) direction or axis produces an offset along the x-axis and any temperature gradient along the East (E)-West (W) direction or axis produces an offset along the y-axis.
Accordingly, it would be desirable to provide a high-precision sensor chip, such as a thermal accelerometer, to minimize the x-axis and/or y-axis offset shift, i.e., the sensitivity to a temperature gradient, due to internally produced as well as externally produced on chip temperature imbalances.